Blew another damn transformer on my Trane XB80

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On 4/19/2011 9:05 AM, Steve Turner wrote:

Steve,
I am (I think) the person who originally suggested getting the inline fuse holder and fuses from Radio Shack, and am glad to see you are pursuing this. Since you have a disconnected secondary, and a fused primary, and the transformer blows the 1/4 amp fuse, it certainly sounds like the primary supply voltage is wrong. Have you actually measured the primary voltage and found it to be about 110 VAC? If so, then the transformer, with no secondary load, must be defective if it immediately blows a 1/4 amp fuse with no secondary load, unless for some reason your logic board is providing some odd waveform other than purely alternating 60 Hz power. Perhaps, for example, some big DC voltage is also present?
It is very hard to believe that the several transformers you have tried are ALL defective. I am way more suspicious of the supply voltage.
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On 4/19/2011 9:51 AM, Smarty wrote:

Yes, the supply voltage is correct (actually, it's 120V, which is what pretty much every other "115V" circuit in the house is receiving). To be safe this time, I ordered more than one replacement transformer (they were cheap) and I just hooked another one to a completely different circuit (on my workbench) with a 1/2-amp fast-acting fuse on the input side and no load on the secondary, and the fuse held and I got a constant 26V on the output side. I will be going to procure some more 1/4-amp fuses (both fast and slow blow) and I will remove the exact transformer I intend to use from the blower unit (it has all the right connectors attached to the wires) and attempt the same experiment on my workbench using both 1/2-amp and 1/4-amp fuses just to be sure I can completely rule out the transformer. If that works, I will take that same transformer back to the blower unit and run the same test there, bypassing the circuit board entirely. More to come...
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On 4/19/2011 11:06 AM, Steve Turner wrote:

Sounds good!
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(brevity snip)
I'm back. The saga continues.
Ok, I got some more 1/4-amp fast-acting fuses (sorry trader4; neither Radio Shack nor Home Depot had any slow-blow fuses rated less than 1.5 amps). I took the transformer out of the blower unit, brought it to my workbench and tested it there (on a completely different 115V circuit). The 1/4-amp fuse held fine, and I got a solid 26 volts on the output side. So far so good.
Reinstalled it in the blower unit (screwed it to the frame) and unhooked the input lines from the logic board (to eliminate it completely from the equation) and sent them straight into the transformer (with the fuse on the black hot wire) and with my meter measuring for 24V on the output side. Closed the cover interlock switch, and "poof"; the fuse blew. That pretty much tells me something is wrong with the incoming power.
I unhooked the transformer and measured the voltage across the hot and neutral wires with the interlock switch closed: 120V. I opened the switch, and the meter still wants to read *something*, like about a volt or so. What the hell? I metered on the *other* side of the interlock switch: nothing. So I hooked the transformer back up (it's still bolted to the frame) with the switch bypassed, and PRESTO; the fuse holds and I have 26V on the output side of the transformer. Bad interlock switch? I wondered if it was shorting to the frame, but it's completely encased in plastic. It seems to meter out fine, and I was going to try to open it up to see if I could find any problem, but it's completely sealed so I gave up.
Anyway, the switch looks like the culprit, so I turn off the breaker at the panel, hook everything back up, reconnecting all the wires the way they are supposed to be, with the exception that the interlock switch is bypassed (not really comfortable with that) and the 1/4-amp fuse still installed on the hot line going into the transformer. Turn the breaker back on, and "poof", the fuse blows again. What the hell? Unhook the transformer output wires from the logic board to remove any load (and back up to the meter to check for 24V output), install a new fuse and repeat; the fuse blows AGAIN, and now I'm just confused. At this point, the "something might be shorted to the frame" theory is starting to look like a distinct possibility. Thoughts?
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On 4/19/2011 1:49 PM, Steve Turner wrote:

One more interesting thing I just discovered. trader4 asked if the transformer was connected to the frame. I just went back and took it out of the frame, set it on the plywood floor, then tested it again. Everything was hooked up normally on the input side (the input wires leading into the logic board (interlock switch still bypassed), then in turn leading to the transformer), but I left the output wires unhooked. Applied power (heh, it turns out there is a local switch in the vicinity to do that, rather than me traipsing to the breaker box - duh), and the fuse *held* (not the case when the transformer was installed in the frame). Switch off, connected the meter to the output leads, switch on; fuse held, 26 volts of output. Switch off, connected the output leads to the logic board, switch on; momentary red LED, then "poof", the fuse blows.... Houston, we have a problem.
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Ok, now I'm just depressed. I could not reproduce the scenario of the fuse blowing when the transformer was mounted to the frame. I traced every last bit of wiring leading to the transformer input; I loosened it from its mounting clamps where it feeds into the junction box, and I separated all wires so none were touching each other or the frame, and I tightened the wire nuts. There are no breaches anywhere, and there just isn't any place where the input wiring can possibly be shorting to the frame of the blower unit.
I reached the point where the only time the fuse blows is when I plug the 24V output wires into the logic board, and since the new logic board arrived earlier today I decided to go ahead and do the swap. Careful labeling of all wires, and careful reading of the documentation, and I know the board is installed correctly. This board is a newer model, and it has an interesting feature:
"The 50A55 has only one serviceable part - an automotive type fuse, which protects the low voltage transformer from damage if its output is short-circuited. If the fuse has opened up, remove whatever caused the short circuit and replace the fuse with only a 3 Amp automotive type fuse. If the fuse is not the cause of the control's problem, replace the entire 50A55 control. There are no other user serviceable parts."
This is the installation manual for the new board: http://www.emersonclimate.com/Documents/White-Rodgers/instruction_sheets/0037-6265.pdf
So guess what happened when I applied power after installing the board? Yep, the on-board 3-amp fuse popped. The 1/4-amp fuse I left on the input side did not.
So the more I piddle-fart around, the more apparent it becomes that I have an electrical problem like none I've personally encountered before. I love fixing stuff myself (it's how I was raised) and I love the challenge of tracking down problems, but now I'm growing weary of the chase...
If you're one of those guys waiting in the wings to say "I TOLD you you should have called in a professional!"... Yeah yeah; save it. :-) I've got some friends of friends (and family too) that are electricians by trade that I can lure in with the promise of beer, and if I need to call one in I will be right by their side shadowing their every move, and I will be *learning* from the experience, not just throwing some money at a local service company.
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wrote:

Again, what is the rating of the transformer? The above would suggest that it's at least 72VA and that in turn means that the 1/4 amp fuse on the primary is way to small. Combine that with that it's fast acting and that could be why you're seeing primary fuses blow, adding to the confusion. If they have a 3A on the 24V side, I'd put a 3/4 amp one on the primary, or maybe even a 1 amp temprarily so you can get the thing debugged. Why the 1/4 primary would work on the bench, but not with the same transformer with no load in the furnace, remains a mystery.
At this point, I'd get the bigger fuse in the primary and hopefully it will stop blowing. Then I'd be looking for what else the logic board drives with 24V and start disconnecting those wires one by one until it stops blowing fuses. My best guess would be that something else it supplies with 24v, ie contactor relay, etc is bad, or that a 24v wire is shorting out somewhere. Did you try disconnecting the thermostat wires, for example?

I think if you called a professional you could very well be at the same point, possibly with an even bigger bill for other parts and certainly lots of labor.
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On 4/19/2011 7:12 PM, snipped-for-privacy@optonline.net wrote:

AHA! - progress. Yes, I did do that, but what I did not do was to also remove the compressor contactor wires when I hooked them back up. One of those (a white wire) connects to the "C" terminal on the thermostat block, the other (a red wire) connects to the "Y" terminal (along with the yellow wire leading to the thermostat). I disconnected those white and red wires (leaving the others from the thermostat connected) and BINGO; the board comes up, the relays pick and the blower motor comes on! Obviously there is a problem in the wiring leading to the compressor, but now at least I know where the hell the problem IS. Thanks for all the help!
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On 4/19/2011 7:45 PM, Steve Turner wrote:

And.... BINGO. I followed the two wires from the thermostat block which clearly led to the outside compressor unit. However, when I popped the service cover on the outside unit, I saw no evidence of those two wires. It was only when I peeked with a flashlight that I could see they were hidden by a secondary access panel (which I've yet to remove), but I *could* see they were connected to larger yellow and blue wires that led directly to the compressor relay. I concluded that the 24V signal was not able to pick the relay for some reason, but actuating it by hand (with a screwdriver, actually) proved that the contactor moves freely (no binding), and the unit comes on when the relay is manually picked. Hmm. I started moving and jiggling those wires and I could tell they had fallen down into the unit and were probably resting on something. When I pulled them upwards and peeked some more, I could clearly see that the larger yellow and blue wires were completely melted through! It looks like they had come to rest on some portion of the condenser coils. It's dark now so I can't do much to repair them, but hopefully with the light of day and some time I'll be back in business!
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On Tue, 19 Apr 2011 20:41:21 -0500, Steve Turner

I fully expect a fairy tale happy ending here. And with babies- twin control boards! You've brought AHR together as one big happy family. Thanks!
--Vic
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On 4/19/2011 9:05 PM, Vic Smith wrote:

I was certainly surprised and thankful for all the participation. Glad I could provide some entertainment. :-)
Yeah, I think I mentioned elsewhere that in a previous home I also had Trane A/C units with White Rodgers control boards and one of them went bad. It certainly won't hurt to have an extra on hand.
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On 4/19/2011 6:41 PM Steve Turner spake thus:
[...]

And thus endeth our saga. Pretty exciting conclusion there. Seriously; I was starting to wonder if you'd ever get this one figured out. This post ought to be put on a plaque somewhere. Maybe print it up like a certificate and post it next to your furnace?
So much for those wild-ass guesses in the thread ("could be a magnet causing a saturated transformer core", etc., etc.)
--
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On 4/19/2011 9:11 PM, David Nebenzahl wrote:

Yeah, no kidding! A simple damn problem that was one *hell* of a bugger to find.

Well I tried to stay on some sort of course that at least made sense to me, but I certainly appreciate all the input no matter how far off base some of it turned out to be. :-)
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On 4/19/2011 8:41 PM, Steve Turner wrote:

The culprits:
http://www.flickr.com/photos/bbqboyee/5637228899/in/photostream
Not sure how it happened, but those two wires are completely fused together. I cut away the nasty business, reconnected them to the signal wires from the logic board and positioned them so they can't ever argue with each other again. The air conditioner is happily plodding along, and everyone lived happily ever after.
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wrote:

What 120V current does the transformer draw when it's on the bench and not driving anything?

Let me make sure I understand this. Normally the 120V goes from the furnace junction box, through the door switch, then to the logic board. From there the logic board in turn sends 120V to the transformer and also gets 24V back from it via the low voltage connections. Correct?
And what you did above as a test was to instead connect the transformer directly so that the 120V goes from junction box, to door switch, to transformer, with the secondary not connected to anything.
Correct?

That''s probably just due to the high impedance of the meter.


Confused here. The other side of the interlock is connected right to the incoming AC junction box, right? So it should have 120V.
>So I hooked

It is very strange. Even though it may not be the real problem, one thing I would do is use bigger fuses. I don't know what the transformer is rated at, but using 2X fuses at least for a few minutes to do some testing should not burn out the transformer. Then you could at least get it to the point you could do some voltage and current measurements, expecially on the secondary.
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On Tue, 19 Apr 2011 13:49:05 -0500, Steve Turner

Connect the wires to the transformer, without the transformer screwed down (or contacting the "ground" of the furnace), and check for voltage between the transformer frame and the furnace frame (ground). There should be less than a volt (ideally, nothing, but a digital meter can read phantom voltage). If there is no voltage, the PRIMARY is not grounding. Measure voltage from each secondary wire to the transformer case. There should be NONE. (Again, a small "phantom" may appear, but touching both leads with your fingers at the same time should eliminate that)
When testing the primary voltage, you ARE closing the lockout switch, right?? And measuring from black to white?
How about measuring from black to ground, and white to ground, with and without the lockout switch closed??
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On Tue, 19 Apr 2011 10:06:40 -0500, Steve Turner

Connect the transformer that tested OK on the bench to the furnace unit and see what happens - - - - - .
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He's talking about it blowing without the transformer connected to anything except the AC. And I doubt the logic board provides the power, typically it gets it's power from power from the transformer which is wired directly to the incoming AC line.

I believe he said he measured it in the past and it was 120V. Is the new transformer fastened to the metal cabinet? I'd seperate it from the cabinet and see if there is any voltage between the two and see if it still blows the fuse. My first thought though is the fuse may be close to the limit of the transformer and being fast-blow it might be blowing just on some initial start-up current.
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Except the logic board is only on the secondary and does not provide power to the primary.
He DOES need to check the primary voltage, and also check for DC.
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On Apr 19, 8:22pm, snipped-for-privacy@snyder.on.ca wrote:

He has checked the primary voltage and reported that it's 120v. The DC component on the primary is an interesting possibility, but where that would be coming from isn't clear. However he also reported that with the thing hooked up to the new logic board, which has a 3A 24V fuse on it, that fuse blew. DC on the primary wouldn't be blowing the 3A secondary fuse. One thing is for sure, with a 3A secondary fuse he should go up to at least a 3/4 amp, if not 1 amp fuse on the primary for testing so that he can at least take some measurements.
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